Process for producing stereospecific polymers

ABSTRACT

Supported stereospecific catalysts and processes for the stereotactic propagation of a polymer chain derived from ethylenically unsaturated monomers which contain three or more carbon atoms or which are substituted vinyl compounds, such as styrene and vinyl chloride. One application is the stereospecific propagation of C 3 -C 4  alpha olefins, particularly the polymerization of propylene to produce syndiotactic polypropylene over a supported metallocene catalyst comprising a stereospecific metallocene catalyst component incorporating a metallocene ligand structure having two sterically dissimilar cyclopentadienyl ring structures coordinated with the central transition metal atom. Both of the cyclopentadienyl groups are in a relationship with one another by virtue of bridge or substituent groups, which provide a stereorigid relationship relative to the coordinating transition metal atom to prevent rotation of said ring structures. The metallocene catalyst component and a co-catalyst component, an alumoxane or an alkyl aluminum, are supported on a particulate silica support comprising spheroidal silica particles having an average diameter within the range of 5-40 microns and an average effective pore size within the range of 50-350 angstroms. The particulate silica support contains at least 50 wt. % of the supported catalyst component within the pore volume of the silica support.

FIELD OF THE INVENTION

[0001] This invention relates to supported stereorigid metallocenecatalysts useful in the production of stereospecific polymers fromethylenically unsaturated compounds and, more particularly, to suchcatalysts incorporating crystalline silica supports and their use.

BACKGROUND OF THE INVENTION

[0002] Numerous catalyst systems for use in the polymerization ofethylenically unsaturated monomers are based upon metallocenes.Metallocenes can be characterized generally as coordination compoundsincorporating one or more cyclopentadienyl groups (which may besubstituted or unsubstituted) coordinated with a transition metal.Various types of metallocenes are known in the art. They includebicyclic coordination compounds of the general formula:

(Cp)₂MeQn  (1)

[0003] characterized by the isospecific metallocenes as described belowand dicyclopentadienyl compounds of the general formula:

CpCp′MeQn  (2)

[0004] characterized by the syndiospecific metallocenes described below.In the aforementioned formulas the Me denotes a transition metal and Cpand Cp′ each denote a cyclopentadienyl group which can be eithersubstituted or unsubstituted with Cp′ being different from Cp, Q is analkyl or other hydrocarbyl or a halo group and n is a number within therange of 1-3. The cyclopentadienyl groups are in a stereorigidrelationship normally provided by a bridged structure between themetallocene rings (not shown in Formulas (1) and (2) above) althoughstereorigidity can be provided through substituent groups which resultin steric hindrance, as described, for example, in U.S. Pat. No.5,243,002 to Razavi. Also, while bridged metallocenes normallyincorporate two cyclopentadienyl groups (or substituted cyclopentadienylgroups), bridged metallocenes incorporating a single cyclopentadienylgroup which is bridged to a heteroatom aromatic group (both beingcoordinated with a transition metal) are also known in the art. Forexample, U.S. Pat. No. 5,026,798 to Canich disclosesdimethylsilyl-bridged cyclopentadienyl—anilino or other heteroatomligand structures with coordination to the transition metal beingprovided through the nitrogen atom of the anilino group.

[0005] As noted previously, isospecific and syndiospecific metallocenecatalysts are useful in the polymerization of stereospecific propagationof monomers. Stereospecific structural relationships of syndiotacticityand isotacticity may be involved in the formation of stereoregularpolymers from various monomers. Stereospecific propagation may beapplied in the polymerization of ethylenically unsaturated monomers suchas C₃+ alpha olefins, 1-dienes such as 1,3-butadiene, substituted vinylcompounds such as vinyl aromatics, e.g., styrene or vinyl chloride,vinyl chloride, vinyl ethers such as alkyl vinyl ethers, e.g., isobutylvinyl ether, or even aryl vinyl ethers. Stereospecific polymerpropagation is probably of most significance in the production ofpolypropylene of isotactic or syndiotactic structure.

[0006] The structure of isotactic polypropylene can be described as onehaving the methyl groups attached to the tertiary carbon atoms ofsuccessive monomeric units falling on the same side of a hypotheticalplane through the main chain of the polymer, e.g., the methyl groups areall above or below the plane. Using the Fischer projection formula, thestereochemical sequence of isotactic polypropylene is described asfollows:

[0007] In FIG. 3 each vertical segment indicates a methyl group on thesame side of the polymer backbone. Another way of describing thestructure is through the use of NMR. Bovey's NMR nomenclature for anisotactic pentad as shown above is . . . mmm . . . with each “m”representing a “meso” dyad, or successive pairs of methyl groups on thesame said of the plane of the polymer chain. As is known in the art, anydeviation or inversion in the structure of the chain lowers the degreeof isotacticity and crystallinity of the polymer.

[0008] In contrast to the isotactic structure, syndiotactic propylenepolymers are those in which the methyl groups attached to the tertiarycarbon atoms of successive monomeric units in the chain lie on alternatesides of the plane of the polymer. Syndiotactic polypropylene in usingthe Fisher projection formula can be indicated by racemic dyads with thesyndiotactic pentad rrrr shown as follows:

[0009] Here, the vertical segments again indicate methyl groups in thecase of syndiotactic polypropylene, or other terminal groups, e.g.chloride, in the case of syndiotactic polyvinyl chloride, or phenylgroups in the case of syndiotactic polystyrene.

[0010] Syndiotactic polymers are semi-crystalline and, like theisotactic polymers, are insoluble in xylene. This crystallinitydistinguishes both syndiotactic and isotactic polymers from an atacticpolymer, which is non-crystalline and highly soluble in xylene. Anatactic polymer exhibits no regular order of repeating unitconfigurations in the polymer chain and forms essentially a waxyproduct.

[0011] Yet another polymer configuration which has both isotactic andatactic features is exemplified by hemi-isotactic polypropylene.Hemi-isotactic polypropylene is characterized by every other methylgroup being on the same side of the polymer with the remaining methylgroups randomly being on the same side or on the opposite side of thepolymer backbone. Hemi-isotactic polypropylene can be characterized bythe following Fisher projection formula in which, as indicated by thebroken lines, alternate methyl groups have random stearicconfigurations.

[0012] Thus, as shown in Structure 5, the methyl groups indicated by thesolid lines are in a mesa relationship with one another, with thealternating methyl groups indicated by the broken lines being randomlyconfigured. Hemi-isotactic polypropylene, while having a semi-orderedstructure, is primarily non-crystalline because of the disorder of thealternate methene units.

[0013] In most cases, the preferred polymer configuration will be adominantly isotactic or syndiotactic polymer with very little atacticpolymer. Catalysts that produce isotactic polyolefins are disclosed inU.S. Pat. Nos. 4,794,096 and 4,975,403. These patents disclose chiral,stereorigid metallocene catalysts that polymerize olefins to formisotactic polymers and are especially useful in the polymerization ofhighly isotactic polypropylene. As disclosed, for example, in theaforementioned U.S. Pat. No. 4,794,096, stereorigidity in a metalloceneligand is imparted by means of a structural bridge extending betweencyclopentadienyl groups. Specifically disclosed in this patent arestereoregular hafnium metallocenes which may be characterized by thefollowing formula:

R″(C₅(R′)₄)₂HfQp  (6)

[0014] In formula (7), (C₅(R′)₄) is a cyclopentadienyl or substitutedcyclopentadienyl group, R′ is independently hydrogen or a hydrocarbylradical having 1-20 carbon atoms, and R″ is a structural bridgeextending between the cyclopentadienyl rings. Q is a halogen or ahydrocarbon radical, such as an alkyl, aryl, alkenyl, alkylaryl, orarylalkyl, having 1-20 carbon atoms and p is 2.

[0015] Catalysts that produce syndiotactic polypropylene or othersyndiotactic polyolefins and methods for the preparation of suchcatalysts are disclosed in the aforementioned U.S. Pat. No. 4,892,851.These catalysts are also bridged stereorigid metallocene catalysts, but,in this case, the catalysts have a structural bridge extending betweendissimilar cyclopentadienyl groups and may be characterized by theformula:

R″(CpR_(n))(CpR′_(m))MeQ_(k)  (7)

[0016] In formula (7), Cp represents a cyclopentadienyl or substitutedcyclopentadienyl ring, and R and R′ represent hydrocarbyl radicalshaving 1-20 carbon atoms. R″ is a structural bridge between the ringsimparting stereorigidity to the catalyst. Me represents a transitionmetal, and Q a hydrocarbyl radical or halogen. R′_(m) is selected sothat (CpR′_(m)) is a sterically different substituted cyclopentadienylring that (CpR_(n)). In formula (8) n varies from 0-4 (0 designating nohydrocarbyl groups, i.e., an unsubstituted cyclopentadienyl ring), mvaries from 1-4, and k is from 0-3. The sterically differentcyclopentadienyl rings produce a predominantly syndiotactic polymerrather than an isotactic polymer.

[0017] Specifically disclosed in U.S. Pat. No. 4,892,851, are bridgedmetallocene ligands having a dissimilar cyclopentadienyl group resultingfrom the reaction of 6,6 dimethyl fulvene with a substitutedcyclopentadiene, fluorene, to produce a ligand characterized by anisopropylidene bridge structure. Preferably, this ligand structure ischaracterized as having bilateral symmetry such as indicated by theisopropylidene(cyclopentadienyl fluorenyl) structure as shown below:

[0018] As indicated by Formula (8), the bilateral symmetry of the ligandstructure is indicated by the balanced orientation about the broken linerepresenting a plane of symmetry extending generally through the bridgestructure and the transition metal atom.

[0019] The foregoing structure may be contrasted with a metallocenewhich lacks bilateral symmetry which can be used in the production ofhemi-isotactic polypropylene as described in the U.S. Pat. No. 5,036,034to Ewen. An example of a compound indicating a lack of bilateralsymmetry is isopropylidene (3-methyl cyclopentadienyl-1 fluorenyl)zirconium dichloride having the ligand structure shown by the followingformula:

[0020] As explained in more detail in the aforementioned Ewen '034patent, the lack of bilateral symmetry is indicated by the right side ofthe structure relative to the broken line being different from the leftside because of the methyl group substituted at the distal position onthe cyclopentadienyl group.

[0021] The various metallocene structures as described above can be usedeither as so-called “neutral metallocenes” in which case an alumoxane,such as methylalumoxane, is used as a co-catalyst, or they can beemployed as so-called “cationic metallocenes” which incorporate a stablenon-coordinating anion and normally do not require the use of analumoxane. Syndiospecific cationic metallocenes are disclosed forexample in U.S. Pat. No. 5,243,002 to Razavi. As disclosed there, themetallocene cation is characterized by the cationic metallocene ligandhaving sterically dissimilar ring structures which are joined to apositively-charged coordinating transition metal atom. The metallocenecation is associated with a stable non-coordinating counter-anion.

[0022] The aforementioned Razavi '002 patent also discloses theestablishment of a stereorigid relationship imparted by asterically-hindered relationship between substituted cyclopentadienylrings which prevent rotation of the ring structures about theircoordination axis. Alternatively, the cyclopentadienyl groups may behighly substituted such that a relatively low kinetic energy state isinduced by the substituents in order to prevent rotation rings abouttheir coordination axis at the temperature of the catalyst. Suchcationic metallocenes also may, of course, like their neutralcounterparts, be characterized by a stereorigid relationship imparted bymeans of a structural bridge between the cyclopentadienyl groups.

[0023] U.S. Pat. No. 5,225,500 to Elder et al discloses stereorigidcationic metallocenes, including, inter alia, bridged metallocenecatalysts useful for the production of syndiotactic polymers. Thebridged metallocene catalysts of U.S. Pat. No. 5,225,500 comprise anunbalanced metallocene cation and a stable, non-coordinatingcounteranion for the metallocene cation. The metallocene cation ischaracterized by a cationic metallocene ligand having stericallydissimilar ring structures joined to a positively charged coordinatingtransition metal atom. The dissimilar cyclopentadienyl rings, at leastone of which is substituted, are both in a stereorigid relationshiprelative to the coordinating metallocene of the metal atom catalyst,and, as noted previously, the stereorigid relationship may be impartedby means of a structural bridge between the dissimilar cyclopentadienylrings.

[0024] While metallocene catalysts are often used as homogeneouscatalysts, it is also known in the art to provide supported metallocenecatalysts. As disclosed in U.S. Pat. Nos. 4,701,432 and 4,808,561, bothto Welborn, a metallocene catalyst component may be employed in the formof a supported catalyst. As described in the Welborn '432 patent, thesupport may be any support such as talc, an inorganic oxide, or aresinous support material such as a polyolefin. Specific inorganicoxides include silica and alumina, used alone or in combination withother inorganic oxides such as magnesia, titania, zirconia and the like.Non-metallocene transition metal compounds, such as titaniumtetrachloride, are also incorporated into the supported catalystcomponent. The inorganic oxides used as support are characterized ashaving an average particle size ranging from 30-600 microns, preferably30-100 microns, a surface area of 50-1,000 square meters per gram,preferably 100-400 square meters per gram, a pore volume of 0.5-3.5cc/g, preferably about 0.5-2 cc/g. Generally, the particle size, surfacearea, pore volume, and number of surface hydroxyl groups are said to benot critical to the Welborn procedure. Specifically disclosed in Welbornis a catalyst in which bis(cyclopentadienyl)zirconium dichloride(unbridged metallocene) is supported on a high surface area silicadehydrated in dry nitrogen at 600° C. and characterized as Davison 952.The Welborn '561 patent discloses a heterogeneous catalyst which isformed by the reaction of a metallocene and an alumoxane in combinationwith the support material. The support in Welborn '561 is describedsimilarly as the support in the Welborn '432 patent.

[0025] A catalyst system embodying both a homogeneous metallocenecomponent and a heterogeneous component, which may be a “conventional”supported Ziegler-Natta catalyst, e.g. a supported titaniumtetrachloride, is disclosed in U.S. Pat. No. 5,242,876 to Shamshoum etal.

[0026] Various other catalyst systems involving supported metallocenecatalysts are disclosed in U.S. Pat. No. 5,308,811 to Suga et al andU.S. Pat. No. 5,444,134 to Matsumoto. In both patents the supports arecharacterized as various high surface area inorganic oxides or clay-likematerials. In the patent to Suga et al, the support materials arecharacterized as clay minerals, ion-exchanged layered compounds,diatomaceous earth, silicates, or zeolites. As explained in Suga, thehigh surface area support materials should have volumes of pores havingradii of at least 20 angstroms. Specifically disclosed and preferred inSuga are clay and clay minerals such as montmorillonite. The catalystcomponents in Suga are prepared by mixing the support material, themetallocene, and an organoaluminum compound such as triethylaluminum,trimethylaluminum, various alkylaluminum chlorides, alkoxides, orhydrides or an alumoxane such as methylalumoxane, ethylalumoxane, or thelike. The three components may be mixed together in any order, or theymay be simultaneously contacted. The patent to Matsumoto similarlydiscloses a supported catalyst in which the support may be provided byinorganic oxide carriers such as SiO₂, Al₂O₃, MgO, ZrO₂, TiO₂, Fe₂O₃,B₂O₂, CaO, ZnO, BaO, ThO₂ and mixtures thereof, such as silica alumina,zeolite, ferrite, and glass fibers. Other carriers include MgCl₂,Mg(0-Et)₂, and polymers such as polystyrene, polyethylene,polypropylene, substituted polystyrene and polyarylate, starches, andcarbon. The carrier has a surface area of 1-1000 m²/g, preferably 50-500m²/g, a pore volume of 0.1-5 cm³g, preferably 0.3-3 cm³/g, and aparticle size of 20-100 microns.

[0027] Of the various inorganic oxides used as supports, silica, in oneform or another, is widely disclosed as a support material formetallocene catalysts. Silica, characterized as Davison D-948 or DavisonD-952, appears as a conventional metallocene support. For example, U.S.Pat. No. 5,466,649 to Jejelowo discloses the use of dehydrated DavisonD-948 silica as a support for various unbridged metallocenes used inconjunction with supported co-catalysts. U.S. Pat. No. 5,498,581 toWelch et al discloses silica for use as a support for either bridged orunbridged metallocenes in which the silica is treated with carbonmonoxide, water, and hydroxyl groups to inactive species. Specificallydisclosed is the silica, Davison D-948, having an average particle sizeof 50 microns. Other silica-based supports are disclosed in U.S. Pat.No. 5,281,679 to Jejelowo, U.S. Pat. No. 5,238,892 to Chang, and U.S.Pat. No. 5,399,636 to Alt. The Chang and Jejelowa patent disclose theuse of a silica support identified as Davison D-948, which ischaracterized as a amorphous silica gel containing about 9.7 wt. %water. As described in the Chang and Jejelowa patents, alumoxane isformed directly on the surface of the silica gel by direct reaction ofan alkyl aluminum with silica gel which is undehydrated so as to ensurethe conversion of the quantity of the alkyl aluminum to an alumoxanethat has a high degree of oligomerization. The water-impregnated gel ischaracterized as having a surface range of 10-700 m²/g, a pore volume ofabout 0.5-3 cc/g, and an absorbed water content of from about 10-50 wt.% in the case of the Jejelowa patent and about 6-20 wt. % in the case ofthe Chang patent. The average particle size for the silica is describedin Chang to be from 0.3-100 microns and in Jejelowa from about 10-100microns. After the alumoxane silica gel component has been formed, themetallocene may be added to the wet slurry.

SUMMARY OF THE INVENTION

[0028] In accordance with the present invention, there are providedsupported stereospecific catalysts and processes for the stereotacticpropagation of a polymer chain derived from ethylenically unsaturatedmonomers which contain three or more carbon atoms or which aresubstituted vinyl compounds, such as styrene and vinyl chloride. Thepreferred application of the present invention is in the stereospecificpropagation of C₃-C₄ alpha olefins, particularly the polymerization ofpropylene to produce syndiotactic polypropylene. In carrying out thepresent invention, there is provided a supported metallocene catalystcomprising a stereospecific metallocene catalyst component and aco-catalyst component comprising at least one of an alkyl alumoxane andan alkylaluminum compound. The metallocene catalyst compoundincorporates a metallocene ligand structure having two stericallydissimilar cyclopentadienyl ring structures coordinated with the centraltransition metal atom. At least one of the cyclopentadienyl ringstructures is a substituted cyclopentadienyl group which provides anorientation with respect to said transition metal atom which issterically different from the orientation of the other cyclopentadienylgroup. Both of the cyclopentadienyl groups are in a relationship withone another by virtue of bridge or substituent groups, which provide astereorigid relationship relative to the coordinating transition metalatom to prevent rotation of said ring structures. Both the metallocenecatalyst component and the co-catalyst component are supported on aparticulate silica support comprising spheroidal silica particles havingan average diameter within the range of 5-40 microns and an averageeffective pore size within the range of 50-350 angstroms. Theparticulate silica support contains at least 50 wt. % of the supportedcatalyst component within the pore volume of the silica support. Thissupported catalyst is contacted in a polymerization reaction zone withan ethylenically unsaturated monomer which contains 3 or more carbonatoms or which is a substituted vinyl compound under polymerizationconditions to produce syndiospecific polymerization of the monomer.

[0029] In a preferred embodiment of the invention, a supportedmetallocene catalyst incorporates a particulate silica support having anaverage diameter and effective pore size as described previously. Theparticulate spheroidal particles have at least 50% of the surface areathereof contained within the pore volume of the support particles. Astereospecific metallocene is supported on the silica support particles.In one application of the invention, the metallocene is an unbalancedmetallocene having a ligand structure in which stereorigidity isimparted by means of a structural bridge extending between dissimilarcyclopentadienyl groups. The metallocene ligand structure has a kineticdiameter which is substantially less than the average pore size of thesilica. The metallocene is prefentially is carried within the interiorpore volume of the silica particles to provide at least 50 wt. % of thepolymerization sites provided on the transitional metal atom within theinterior pore volume of the support.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030]FIG. 1A is an elevational view of an idealized depiction of asolid spheroidal catalyst particle which can be used in carrying out thepresent invention.

[0031]FIG. 1B is a side elevational view showing an idealized depictionof another form of a generally spherical catalyst particle which can beemployed in carrying out the present invention.

[0032]FIG. 1C is a transverse side elevational view of the catalystparticle of FIG. 1B.

[0033]FIG. 1D is a side elevational view of an idealized depiction of amodified form of a catalyst particle corresponding to the catalystparticle of FIGS. 1B and IC.

[0034]FIG. 2 is a photograph of angular catalyst particles of aconfiguration used in the prior art to support metallocene catalysts.

[0035]FIGS. 3 and 4 are photographs of spheroidal catalyst particlesgenerally conforming to the catalyst particle ideally depicted in FIG.1A.

[0036]FIGS. 5, 6, and 7 are photographs of catalyst support particlesgenerally conforming to the support particles ideally depicted in FIGS.1B through 1D.

DETAILED DESCRIPTION OF THE INVENTION

[0037] The present invention involves processes for supportedstereospecific metallocenes which may be neutral or cationic, and whichare effective in stereospecific polymer propagation, especiallysyndiotactic polymer propagation, to provide polymer structure havingreduced gel imperfections. As indicated previously, the metallocene-typecatalysts, like the more traditional Ziegler-Natta catalysts, can besupported on various inorganic supports. When supporting traditionalZiegler-Natta-type catalysts, such as zirconium, hafnium, and titaniumtetrahalides on crystalline materials, such as magnesium dichloride, itis conventional to “activate” the support material to provide particlesof a large surface area on which the transition metal sites are exposedfor the polymerization reaction in which the monomer units areincorporated into the growing polymer chain. Similar thinking hasdominated the use of supports for metallocene catalysts. Thus, highsurface area materials are thought to be desirable and where microporousmaterials are employed, silica being the principal one of such material,relatively large pore spaces in relationship to the kinetic size of themetallocene ligand are thought to be desirable. Thus, as explained inthe aforementioned patent to Suga et al, the pore volume of the supportmaterial should be provided by pores having a radii of at least 20angstroms. Since all things being equal, a higher pore volume correlateswith a lower surface area, the traditional silca supports are able toprovide relatively high surface areas with acceptable pore volumesbecause of the irregular angular nature of the silica particles. Asdescribed in greater detail below, such silica particles can becharacterized as highly irregular polygons with many irregular surfacefacets and acute angles between intersecting surfaces.

[0038] The present invention relates to silica-supported stereospecificmetallocenes catalysts which result in a very substantial reduction inimperfections in the resulting polymer product obtained with thesupported metallocene catalyst. The silica support employed in thepresent invention is characterized as a silica having generallyspheroidal particles with a relatively small average diameter and arelatively high pore volume of about 1 ml./gr., preferably 1.2 ml./gr.and more preferably within the range of 1.3-1.5 ml./gr.

[0039] The preferred form of silica support is characterized bygenerally spheroidal silica particles having generally axialdepressions, sometimes extending completely through the particle, toprovide a “donut” configuration to the particle. While solid spheroidalparticles provide a substantially-enhanced film configuration havingreduced imperfections, similar spheroidal-shaped particles with the“donut” configuration provide a marked decrease in imperfections withrespect to those polymers produced by catalysts supported on the smoothspherical-type silica particles.

[0040] The particulate silica support employed in the present inventioncomprises spheroidal silica particles having an average diameter withinthe range of 5-40 microns and, more preferably, within the range ofabout 10-40 microns. The metallocene catalyst is primarily supportedwithin the pore surface area of the silica, as contrasted with theexternal surface area, with the amount of metallocene supportedexternally of the silica particle accounting for a minor fraction,usually no more than 10 wt. % of the total metallocene found on thesilica support. Stated otherwise, a major fraction of more than 50% andpreferably at least 90 wt. % of the metallocene is contained within thepore volume of the silica support. An improved polymer characterized interms of reduced gel defects is observed for catalysts supported onrelatively high surface areas, small particle size spherical particleshaving an average particle size of about 10 to 25 microns. While thesesolid spheres result in substantially better polymers than thoseproduced by metallocene supported on the more conventional, irregularparticulate silica, much better results are obtained employing silicasupports of the “donut-type” spheroidal configuration. Thissubstantially-reduced number of gel defects is accomplished notwithstanding that the “donut-type” spheroidal silica has a somewhatlarger average particle size than the solid spheroidal silica. WhileApplicants' invention is not to be limited by theory, it is postulatedthat these spheroidal silica particles, particularly the “donut”-shapedspheroidal particles, become highly fractured during the polymerizationprocedure. The fracturing of the spheroidal silica particles not onlycontinuously exposes more transition metal sites to the monomerinsertion mechanism during the polymerization process, it ultimatelyreduces the silica particles to a size such that they do not result insignificant numbers of gel defects. Thus, the “donut”-shaped spheroidalsilica particles, while larger than the more perfectly formed spheroidalsilica particles, are believed ultimately to fracture to a size of aboutfive microns or less, usually about three microns or less, with anattendant highly significant reduction in gel imperfections. While thedonut-shaped configurations are thus preferred, as shown by theexperimental data presented hereinafter, the relatively solid spheroidalparticles, which more nearly approach being perfect spheres, stillresult in substantial reduction in gel imperfections when compared withpolymers prepared using the more conventional irregular silica supports.

[0041] The metallocenes employed in the present invention may beisospecific or syndiospecific catalysts, as described previously, butpreferably are syndiospecific catalysts, and the invention will bedescribed with regard to formation of syndiotactic polyolefins,specifically syndiotactic polypropylene. The term “metallocene,” as usedherein and in accordance with normal art usage, denotes anorganometallic coordination compound in which two cyclo-C₅ ligands(cyclopentadienyl or substituted cyclopentadienyl rings) are bonded to acentral or “sandwiched” metal atom which may be provided by a transitionmetal halide, alkyl, alkoxy, alkoxy halide or the like. Such structuresare sometimes referred to as “molecular sandwiches: since the cyclo-C₅ligands are oriented above or below the plane of the central coordinatedmetal atom. The metallocene catalysts which are supported in accordancewith the present invention may be neutral or they may be cationic. Bythe term “cationic metallocene” is meant a metallocene in which thecentral coordinated metal atom carries a positive charge, that is, themetallocene complex is a cation associated with a stable anion. Theneutral or cationic metallocenes employed in accordance with the presentinvention are stereorigid. Preferably, stereorigidity is imported to theligand structure by virtue of a chemical bridge extending between thecyclopentadienyl (or substituted cyclopentadienyl) rings.

[0042] As noted previously, U.S. Pat. No. 4,892,851 discloses thepreparation of syndiotactic polypropylene or other polyolefins throughthe use of bridged stereorigid metallocene catalysts. The aforementionedU.S. Pat. No. 5,225,500 discloses stereorigid metallocene catalysts,including those in which stereorigidity is imparted by a bridgestructure, in which a neutral metallocene is ionized to provide a stablecationic catalyst. Neutral metallocenes may also be converted to thecationic form following procedures of the type disclosed in theaforementioned U.S. Pat. Nos. 5,243,002 and 5,205,500 and also inEuropean Patents 277,003 and 277,004 to Turner, and further by a processemploying a triphenylcarbenium boronate as discussed in greater detailin U.S. Pat. No. 5,387,568 to Ewen et al or a triphenylcarbeniumaluminate as disclosed in U.S. patent application Ser. No. 893,522 filedJun. 4, 1992, by Elder et al. In the bridged metallocene catalystsemployed in the present invention, the cyclopentadienyl groups may bethe same if they are to be used for isotactic polymer propagation, ordifferent if they are to be used for syndiotactic polymer propagation.

[0043] As noted previously, a preferred application of the presentinvention is in the use of supported syndiospecific catalysts having astereorigid bridge structure extending between dissimilarcyclopentadienyl rings. Such syndiospecific metallocenes may becharacterized by the previously described Formula (7):

R″(CpR_(n))(CpR′_(m))MeQ_(k)  (7)

[0044] In Formula (7), R and R′ are selected such that CpR′_(m) is asterically different ring than CpR_(n). Isospecific catalysts employedin accordance with the present invention may also be characterized byFormula (7), with the proviso that the two cyclopentadienyl groups,which may be substituted or unsubstituted, are chemically the same, thatis, CpR′_(m) is the same as CPR_(n) and m and n may both vary from 0 to4. Such isospecific catalysts can be characterized by the formula:

R″(C₅R′₄)₂MeQ_(k)  (10)

[0045] corresponding to Formula (6) above except that Me is a transitionmetal not limited to hafnium, and more specifically by the formula:

R″(Ind)₂MeQ_(k)  (11)

[0046] wherein Ind is an indenyl or substituted indenyl group in aracemic configuration.

[0047] As noted previously, the stereorigid metallocene catalystsemployed in the present invention may be neutral or cationicmetallocenes. The cationic metallocenes correspond to the structuresdepicted by Formulas (7) and (10) with the exception that k is aninteger from 0 to 2, rather than the transition metal being possiblytrisubstituted, as in the case of the neutral metallocenes. Suchcationic metallocene catalysts may be characterized by the followingformula:

[R″CpR_(n))(CpR′_(m))MeQ_(k)]⁺P—  (12)

[0048] In Formula (12), Cp, R, R′, Me, m, and n are as describedpreviously, k is a number from 0 to 2, and P is a stable noncoordinatingcounter anion. The cationic catalysts of Formula (12) may be preparedfrom the corresponding neutral metallocenes using procedures asdescribed above.

[0049] The counter anion indicated by P in Formula (12) is a compatiblenoncoordinating anion which may be of the type described in theaforementioned Elder et al and Razavi U.S. patents or the TurnerEuropean patents. The anion P either does not coordinate with themetallocene cation or is only weakly coordinated to the cation, therebyremaining sufficiently liable to be displaced by a neutral Lewis base.As described in the Turner patents, the term “compatible noncoordinatinganion” identifies an anion which, when functioning as a stabilizinganion in the metallocene catalyst system, does not transfer an anionicsubstituent or fragment thereof to the cation to form a neutralmetallocene and boron byproduct or other neutral metal or metalloidbyproduct, as the case may be. Suitable noncoordinating anions include:[W(PhF₅)]—, [Mo(PhF₅)—] (wherein PhF₅ is pentafluoryl phenyl), [ClO₄]—,[SbR₆]—, and [AlR₄]— (wherein each R is independently, Cl, a C₁-C₅ alkylgroup, preferably, a methyl group, an aryl group, e.g., a phenyl orsubstituted phenyl group, or a fluorinated aryl group. For a furtherdescription of compatible noncoordinating anions and their associatedcations which may be employed in the present invention, reference ismade to U.S. Pat. Nos. 5,225,500 and 5,243,002, 5,387,568, EPO PatentNos. 277,003 and 277,004, and U.S. patent application Ser. No. 893,522,the entire disclosures of which are incorporated herein by reference.

[0050] The silica-supported bridged metallocene catalysts of the presentinvention may be isospecific or syndiospecific, as discussed previously.The bridge configuration of the R″ structural bridge is controlled bythe terminal carbon substituents of the substituted fulvene. Forexample, where the fulvene reactant is 6,6 dimethyl fulvene, thestructural bridge will be a C₃ alkylene group, commonly referred to aspropylidene. The use of 6,6 methyl, ethyl fulvene will result in a C₄structural bridge, and the use of 6,6 diethyl fulvene as a reactant willresult in a C₅ structural bridge. The use of 6,6 diphenyl fulvene willresult in a diphenyl methylene bridge. Where the bridge is a hydrocarbylgroup, it is preferably selected from the group consisting of alkylradicals having 3-6 carbon atoms, more preferably, 3-5 carbon atoms.Examples of alkyl bridges include propyl, butyl, and pentyl bridgeswhich may be substituted or unsubstituted. Me in Formula (12),preferably, is a Group 4 or 5 metal, and more preferably, a Group 4metal, specifically titanium, zirconium, or hafnium. Vanadium is themost suitable of the Group 5 metals. Q will usually be a methyl or ethylgroup or chlorine.

[0051] Where the present invention is employed in the production ofsyndiotactic polymers, the cyclopentadienyl and substituted fulvenereactants are chosen so that the resulting syndiospecific catalystsexhibit bilateral symmetry of the metallocene ligands when viewed asplanar projections of the cyclopentadienyl groups. By the term“bilateral symmetry,” as used herein, it is meant the symmetry of theligand as viewed through the axes of the substituted or unsubstituted Cpgroups, as shown above by Formula (8). For example, the reaction offluorene with 6,6-dimethyl fulvene produces the isopropylidene(cyclopentadienyl-1-fluorenyl) ligand which exhibits such bilateralsymmetry. However, the similar reaction carried out with a ringsubstituted fulvene, such as 3-alkyl, 6,6-dimethyl fulvene, would resultin a corresponding structure, but with the cyclopentadienyl groupsubstituted at the three position. This structure would not exhibitbilateral symmetry as shown above by Formula (9). The ligand with twoidentical substituents at the 3 and 4 positions on the cyclopentadienylgroup would have bilateral symmetry.

[0052] Usually, in the metallocenes employed in the present invention,Me is titanium, zirconium, hafnium, or vanadium; Q is, preferably, amethyl or halogen, more preferably chorine; and k, preferably, is 2 forneutral metallocenes, and 1 for cationic metallocenes, but may vary withthe valence of the metal atom. Exemplary hydrocarbyl radicals includemethyl, ethyl, propyl, isopropyl, butyl, isobutyl, amyl, isoamyl, hexyl,heptyl, octyl, nonyl, decyl, cetyl, phenyl, and the like. Otherhydrocarbyl radicals include other alkyl, aryl, alkenyl, alkylaryl, orarylalkyl radicals. Further, Rn and R′m may comprise hydrocarbylradicals attached to a single carbon atom in the Cp ring, as well asradicals that are bonded to two carbon atoms in the ring as in the caseof a fluorenyl. Neutral metallocenes may be converted to the cationicstate following procedures as described previously. Exemplary neutralsyndiospecific metallocenes which may be employed in the presentinvention are isobutylidene(cyclopentadienyl-1-fluorenyl), zirconiumdimethyl, isopentylidene (cyclopentadienyl-1-fluorenyl)zirconiumdimethyl, isopropylidene(indenyl) (cyclopentadienyl) zirconium dimethyl,isopropylidene(cyclopentadienyl-1-fluorenyl)zirconium dimethyl, diphenylmethylene(cyclopentadienyl-1-fluorenyl)zirconium dimethyl, and thecorresponding dichlorides or methylchlorides.

[0053] Examples of isospecific neutral metallocenes which can beemployed in accordance with the present invention include isopropylidenebis-(2,3 dimethylcyclopentadienyl) zirconium dimethyl, isopropylidenebis (tetramethylcyclopentadienyl) zirconium dimethyl, and isopropylidenebis (2,4 dimethylcyclopentadienyl) zirconium dimethyl, ethylenebis-(indenyl)zirconium dimethyl and the corresponding dichlorides.Further neutral metallocenes include ethylene bis(2-methyl indenyl)zirconium dichloride, dimethyl silyl bis(2-methyl indenyl) zirconiumdichloride, diphenyl silyl bis(2-methyl indenyl) zirconium dichloride,diphenyl silyl bis(2-methyl, 4-phenyl-indenyl) zirconium dichloride, anddiethyl silyl bis(2-methyl, 4-phenyl indenyl) zirconium dichloride.Other corresponding metallocenes, especially the corresponding hafniumand titanium metallocenes, can also be employed in accordance with thepresent invention to produce syndiospecific or isospecific catalysts.Similarly, other metallocene dialkyls, for example, such as thezirconium or hafnium diethyls and other dihalides, may also be madefollowing the present invention, but, as a practical matter, the neutralmetallocenes will be in the form of dimethyl or dichloride compounds,and the metallocenes will usually be in the form of the chlorides.

[0054] The silica particles of a spheroidal configuration which can beemployed in carring out the present invention are shown schematically inFIGS. 1A, 1B, 1C, and 1D. FIG. 1A illustrates an idealized depiction ofa solid spheroidal catalyst particle 10 corresponding to the highsurface area catalyst identified below as Catalyst A. This catalyst hasa relatively small average particle size and a relatively high surfacearea per gram in comparison to the somewhat larger particle size,“donut” configuration, as illustrated by FIGS. 1B and 1C. As shown inFIG. 1B, the generally spherical silica particle 12 is characterized bya central bore 14 so that the silica particle is of a spherical annularconfiguration. When viewed from the front elevation of FIG. 1B, thesilica particle is generally shown to resemble a donut and, hence, the“donut” configuration. As shown in FIG. 1C, when viewed from a sideelevation, the central bore is not apparent, and the silica particle 12appears to conform generally to the spherical particle of FIG. 1A,although of a somewhat large diameter. FIG. 1D shows a corresponding“donut-type” configuration in which the central bore does not extendcompletely through the catalyst particle 16 but instead forms apronounced depression 17 so that when viewed from the side of thedepression the silica particle is still reminiscent of a “donut-type”configuration.

[0055] The supported metallocenes used in the present invention can beprepared by any suitable technique. Procedures known in the prior artfor preparing silica-supported metallocene catalysts can be used inpreparing the supported metallocenes of the present invention with theexception that the support takes the form of spheroidal silica particleshaving the characteristics called for in the present invention. Thus,procedures such as those disclosed in the aforementioned U.S. Pat. No.5,308,811 to Suga et al U.S. Pat. No. 5,444,134 to Matsumoto et al maybe employed in forming the supported catalysts of the present inventionwith the exception of the surface area and pore size criteria specifiedin these references. In employing the procedures of this nature, thecatalyst components, i.e., the organo aluminum compound, and the silicasupport can be mixed together in any order or contacted simultaneouslyas disclosed, for example, in the Suga et al and Matsumoto references.For a further description of such procedures, reference is made to U.S.Pat. No. 5,308,811 to Suga et al and U.S. Pat. No. 5,444,134 toMatsumoto et al, the entire disclosures which are incorporated herein byreference.

[0056] Preferably where a so-called neutral metallocene is employed itis desirable to first treat the silica support material with analkylalumoxane co-catalyst such as methylalumoxane (MAO) with subsequentcontact of alumoxane treated support with the metallocene. Subsequent tocontact of the molecular support with the alumoxane and metallocene, aco-catalyst such as trimethylaluminum, triethylaluminum ortri-isobutylaluminum (TIBAL) can be added to the silica-supportedcatalyst and the catalyst then used in the polymerization reaction.

[0057] In experimental work carried out respecting the presentinvention, a syndiospecific metallocene catalyst,diphenylmethylene(cyclopentadienyl fluorenyl)zirconium dichloride, wasused in the polymerization of propylene as a homogeneous catalyst and asa supported catalyst on four different silica supports. In each case,methylalumoxane (MAO) was used as the co-catalyst component ionizingagent, and TIBAL was used as a co-catalyst scavenging agent.

[0058] Four silica supports were used in experimental work respectingthe invention. The first, denominated herein as Support A, was anamorphous silica of irregular angular particles of an average particlesize of about 35-40 microns with a surface area of about 470 m²/g. and apore volume of about 0.73 milliliters per gram. A photograph ofparticles of Support A, shown to an enlargement of 100×, is illustratedin FIG. 2.

[0059] The second amorphous angular silica support, denominated hereinas Support B, Q-10, had a somewhat smaller average particle size ofabout 25 microns. This catalyst had a surface area of about 300 m²/g.and a pore volume of about 1.0 ml/g. Two silica supports of the typeused in carrying out the present invention were also used in theexperimental work. The first, Catalyst C, was a highly sphericalcatalyst (without the “donut hole” configuration) of the type depictedin the photographs of FIG. 1A. This catalyst had an average particlesize of 12 microns, a surface area of about 760 ²m/g and a pore volumeof about 0.9 ml/g. The fourth catalyst, Catalyst D, was a spheroidalsilica having the “donut hole” configuration as shown in FIGS. 1B and1C. This support had a larger average particle size and acorrespondingly lower surface area than the silica having the morenearly-perfect spheroidal configuration, as exemplified by Catalyst C.Silica Support D had an average particle size of 25 microns, a surfacearea of about 300 m²/g. and a pore volume of 1.37 ml/g.

[0060] Actual photographs of silica particles corresponding generally tothe idealized configurations of FIGS. 1A-1D are shown in FIGS. 3-7. Asshown in FIGS. 3 and 4, the silica particles conforming to Support Care, in fact, very nearly conformed to almost perfect spheres having insome cases minor imperfections on the outer surface as shown in FIG. 4and in other cases being relatively free of such imperfections. Thesilica particles are shown in FIG. 3 with a magnification of 1000 and inFIG. 4 with magnifications of 5,000.

[0061] Photographs of silica particles of Support D are shown in FIG. 5at a magnification of 100, FIG. 6 (magnification 2000), and FIG. 7(magnification 3,000). As shown in FIG. 5 and also in more detail inFIG. 6, the silica particles of Support D conform in some cases tonearly perfect spheres (with the central-recessed or bore “donut-hole”configuration) to highly irregular configurations which tend to be, insome cases, ellipsoidal and highly fragmented. FIG. 7 shows silicaparticles of Support D which generally are of a spheroidal configurationwith only minor imperfections on the surfaces.

[0062] The four silica supports described above were treated withmethylalumoxane and then treated with the metallocene catalyst. Asolution of methyl alumoxane was added to the silica particles andstirred in refluxed toluene for a period of four hours at 116° C. Theweight ratio of MaO to the silica support was in each case within therange of about 0.7-0.9. The MaO-treated silica was then recovered fromthe toluene solution by filtering, washed three times with toluene, anddried at room temperature over night. A syndiospecific metallocene,diphenylmethylene (cyclopentadienylfluorenyl) zirconium dichloride wasthen added to the silica support in an amount of about 2 wt. %metallocene based upon the silica and stirred at room temperature. Thesolid product was then filtered and washed in water at room temperature.

[0063] The catalysts thus prepared were used in the polymerization ofpropylene to produce syndiotactic polypropylene. The four polymers thusproduced are designated herein as Polymers A, B, C, and D correspondingto the catalyst as supported on Metallocene Supports A, B, C, and D.Thus, Polymer A was prepared using the syndiospecific metallocenesupported on Silica Support A, Polymer B with the same metallocene, onsilica support B and so on. The four polymers produced had similarsyndiotacticities. Two polymers were evaluated in terms of racemicpentads (rrrr) and the total content of racemic diads in the polymerstructure. For Polymer B, produced by polymerization of thediphenylmethylene(cyclopentadienyl fluorenyl) zirconium chloridesupported on Silica B, the polymer structure was characterized by 81.6%racemic pentads and 93.5% racemic diads. The activity of the catalystsupported on Support B was 3,369 grams per gram per hour. The catalystsupported on the preferred Support D had a much higher activity, 16,880grams per gram per hour. Polymer D was characterized by 80.40% racemicpentads and 93.1% racemic diads.

[0064] The Polymers A through D were then used to prepare cast films byprocessing of pellet or powder samples of the polymer by means of ascrew extruder and a t-die. Each polymer was extruded at a temperatureof about 230° C. in the feed zone, and a temperature of about 250° C. inthe compression die zone. The film was formed at a cast roll temperatureof 30° C. to a thickness of 50 microns.

[0065] The gel imperfections, commonly termed “fish-eyes,” were observedunder the naked eye and with the aid of a microscope to classify thefish-eyes into four levels by length. Level A characterized by a lengthof 300 microns or more; Level B with fish-eye lengths within the rangeof 200-300 microns; Level C1 ranging from 100-200 microns and Level C2,less than 100 microns. The films were then characterized to identify gelimperfections of a size less than 200 microns. The results of the filmsformed from the four polymers A-D in terms of gel imperfections per areaunit of 600 square centimeters are set forth in Table I: TABLE I Error!Bookmark not defined. Silica Support Avg. Particle Size, Gel SiO₂Particles Type Microns <200 Microns A 38 3000 B 25 3000 C 12 1500 D 25 45

[0066] As can be seen from an examination of Table I, the sydiotacticpolypropylene produced with metallocenes supported on Support C showedgel imperfections at a rate of about one-half of those produced formetallocenes supported on the conventional granular support. For thesyndiospecific metallocene supported on Support D, the decrease in gelimperfections was remarkable, close to less than 2% of the gelimperfections observed for the conventional silica supports.

[0067] Having described specific embodiments of the present invention,it will be understood that modifications thereof may be suggested tothose skilled in the art, and it is intended to cover all suchmodifications as fall within the scope of the appended claims.

What is claimed:
 1. In a method for the stereotactic propagation of apolymer chain derived from an ethylenically unsaturated monomer, thesteps comprising: a) providing a supported metallocene catalystcomprising a stereospecific metallocene catalyst component and analuminum containing a co-catalyst component comprising at least one ofan alkyl alumoxane and an alkylaluminum compound, said stereospecificmetallocene incorporating a metallocene ligand structure having twosterically dissimilar cyclopentadienyl ring structures coordinated witha central transition metal atom; at least one of said cyclopentadienylring structures being a substituted cyclopentadienyl group whichprovides an orientation with respect to said transition metal atom whichis sterically different from the orientation of the othercyclopentadienyl group with respect to said transition metal atom, andboth of said cyclopentadienyl groups being in a relationship with eachother providing a stereorigid relationship relative to said coordinatingtransition metal atom to prevent rotation of said ring structures, saidmetallocene catalyst component and said co-catalyst component beingsupported on a particulate silica support comprising spheroidal silicaparticles having an average diameter within the range of 5-40 micronsand an average effective pore size within the range of 50-350 angstromsand containing at least 50 wt. % of the supported catalyst componentwithin the pore volume of said particulate silica support; and b)contacting said catalyst in a polymerization reaction zone with anethylenically unsaturated monomer which contains 3 or more carbon atomsor which is a substituted vinyl compound and maintaining said reactionzone under polymerization conditions to produce stereospecificpolymerization of said monomer with an attendant rupturing of saidsilica support to produce secondary silica support particles having anaverage particle size of no more than 12 microns.
 2. The method of claim1 wherein the contact of said catalyst to produce stereospecificpolymerization of said monomer is accompanied with an attendantrupturing of said silica support to produce secondary silica supportparticles having an average particle size of about 5 microns or less. 3.The method of claim 1 wherein said particulate silica support comprisesspheroidal silica particles at least some of which comprise a centralbore extending at least partially through said particle to provide adonut-type configuration.
 4. The method of claim 1 wherein thetransition metal of said metallocene is titanium, zirconium, hafnium, orvanadium.
 5. The method of claim 1 wherein said ethylenicallyunsaturated monomer is a vinyl aromatic compound.
 6. The method of claim1 wherein said ethylenically unsaturated monomer is a C₃-C₄ alphaolefin.
 7. The method of claim 7 wherein said C₃-C₄ alpha olefin ispropylene.
 8. The method of claim 1 wherein said metallocene is a chiralstereorigid metallocene characterized by the formula: R″(C₅(R′)₄)₂MeQpwherein each (C₅(R′)₄) is a substituted cyclopentadienyl ring; each R′is the same or different and is a hydrogen or hydrocarbyl radical having1-20 carbon atoms; R″ is a structural bridge between the two (C₅(R′)₄rings imparting stereorigidity to said catalyst with the two (C₅(R′)₄)rings being in a racemic configuration relative to Me, and R″ isselected from the group consisting of an alkylene radical having 1-4carbon atoms, a silicon hydrocarbyl radical, a germanium hydrocarbylradical, a phosphorus hydrocarbyl radical, a nitrogen hydrocarbylradical, a boron hydrocarbyl radical, and an aluminum hydrocarbylradical; Me is a group 4b, 5b, or 6b metal as designated in the PeriodicTable of Elements; each Q is a hydrocarbyl radical having 1-20 carbonatoms or is a halogen; and 0≦p ≦3.
 9. The method of claim 8 wherein saidethylenically unsaturated monomer is a C₃-C₄ alpha olefin.
 10. Themethod of claim 9 wherein the transition metal of said metallocene istitanium, zirconium, hafnium, or vanadium.
 11. The process of claim 1wherein said metallocene is characterized by the formula:R″(Cp_(a)R_(n))(Cp_(b)R′_(m))MeQ_(p) wherein Cp_(a) is a substitutedcyclopentadienyl ring, Cp_(b) is an unsubstituted or substitutedcyclopentadienyl ring; each R is the same or different and is ahydrocarbyl radical having 1-20 carbon atoms; each R′_(m) is the same ordifferent and is a hydrocarbyl radical having 1-20 carbon atoms; R″ is astructural bridge between the cyclopentadienyl rings impartingstereorigidity to the catalyst and is selected from the group consistingof an alkylene radical having 1-4 carbon atoms, a silicon hydrocarbylradical, a germanium hydrocarbyl radical, a phosphorus hydrocarbylradical, a nitrogen hydrocarbyl radical, a boron hydrocarbyl radical,and an aluminum hydrocarbyl radical; Me is a group 4b, 5b, or 6b metalfrom the Periodic Table of Elements; each Q is a hydrocarbyl radicalhaving 1-20 carbon atoms or is a halogen; 0≦p≦3; 0≦m≦4; 1≦n≦4; andwherein R′_(m) is selected such that (Cp_(b)R′_(m)) is a stericallydifferent ring than (Cp_(a)R_(n)).
 12. The process of claim 11 wherein Ris selected such that (Cp_(a)R_(n)) forms a substituted or unsubstitutedfluorenyl group.
 13. The process of claim 12 wherein Me is titanium,zirconium, hafnium, or vanadium.
 14. The process of claim 13 wherein R″is a methylene, ethylene, organosilyl, substituted methylene, orsubstituted ethylene radical.
 15. The process of claim 14 whereinR″(CpR_(n))(CpR′_(m)) forms anisopropylidene(cyclopentadienyl-1-fluorenyl) radical or adiphenylmethylene(cyclopentadienyl-1-fluorenyl) radical.
 16. The processof claim 13 wherein R is selected such that (Cp_(a)R_(n)) forms asubstituted fluorenyl radical having bilateral symmetry and R_(n) isselected such that (Cp_(b)R_(m)) forms an alkyl substituted orunsubstituted cyclopentadienyl radical having bilateral symmetry. 17.The process of claim 16 wherein R″ is a methylene, ethylene, organomethylene, or organo silyl radical.
 18. The process of claim 1 whereinR″(CpR_(n))(CpR″_(m)) forms anisopropylidene(cyclopentadienyl-1-fluorenyl) radical or a diphenylmethylene(cyclopentadienyl-1-fluorenyl).
 19. A supported metallocenecatalyst comprising: a) a particulate silica support comprisingspheroidal silica particles having an average diameter within the rangeof 5-40 microns and an average effective pore size within the range of50-350 with at least 50% of the surface area of said support beingcontained within the pore volume of said particulate support; b) astereospecific metallocene supported on said particulate support andincorporating a metallocene ligand structure having two stericallydissimilar cyclopentadienyl ring structures coordinated with a centraltransition metal atom; at least one of said cyclopentadienyl ringstructures being a substituted cyclopentadienyl group which provides anorientation with respect to said transition metal atom which issterically different from the orientation of the other cyclopentadienylgroup with respect to said transition metal atom, both of saidcyclopentadienyl groups being in a relationship with each otherproviding a stereorigid relationship relative to said coordinatingtransition metal atom to prevent rotation of said ring structures; andc) said metallocene ligand structure having a kinetic diameter which isless than the average pore size of said silica and being preferentiallycarried within the interior pore volume of said silica particles toprovide at least 50% of the polymerization sites provided on saidtransition metal atom being within the interior pore volume of saidsupport.
 20. The supported metallocene catalyst of claim 19 comprisingspheroidal silica particles having an average diameter within the rangeof 10-25 microns.
 21. The catalyst of claim 19 wherein at least some ofsaid spheroidal silica particles comprise a central bore extending atleast partially through said particle to provide a donut-typeconfiguration.
 22. The catalyst of claim 19 wherein at least 90 wt. % ofthe polymerization sites provided on said transition metal atom arelocated within the interior pore volume of said silica supportparticles.
 23. The supported metallocene catalyst of claim 19 furthercomprising an aluminum-containing co-catalyst incorporated into saidparticulate silica support wherein said aluminum-containing co-catalystis incorporated predominantly within the interior of said particulatesupport.
 24. The composition of claim 19 wherein said stereospecificmetallocene is characterized by the formula:R″(Cp_(a)R_(n))(Cp_(b)R′_(m))MeQ_(p) wherein Cp_(a) is a substitutedcyclopentadienyl ring, Cp_(b) is an unsubstituted or substitutedcyclopentadienyl ring; each R is the same or different and is ahydrocarbyl radical having 1-20 carbon atoms; each R′_(m) is the same ordifferent and is a hydrocarbyl radical having 1-20 carbon atoms; R″ is astructural bridge between the cyclopentadienyl rings impartingstereorigidity to the catalyst and is selected from the group consistingof an alkylene radical having 1-4 carbon atoms, a silicon hydrocarbylradical, a germanium hydrocarbyl radical, a phosphorus hydrocarbylradical, a nitrogen hydrocarbyl radical, a boron hydrocarbyl radical,and an aluminum hydrocarbyl radical; Me is a group 4b, 5b, or 6b metalfrom the Periodic Table of Elements; each Q is a hydrocarbyl radicalhaving 1-20 carbon atoms or is a halogen; 0≦p≦3; 0≦m≦4; 1≦n≦4; andwherein R′_(m) is selected such that (Cp_(b)R′_(m)) is a stericallydifferent ring than (Cp_(a)R_(n)).
 25. The composition of claim 24wherein R is selected such that (Cp_(a)R_(n)) forms a substituted orunsubstituted fluorenyl group.
 26. The composition of claim 25 whereinMe is titanium, zirconium, hafnium, or vanadium and R″ is a methylene,ethylene, organosilyl, substituted methylene, or substituted ethyleneradical.
 27. The composition of claim 26 wherein R″(CpR_(n))(CpR′_(m))forms an isopropylidene(cyclopentadienyl-1-fluorenyl) radical or adiphenylmethylene(cyclopentadienyl-1-fluorenyl) radical.
 28. In aprocess for the preparation of a supported metallocene catalyst, thesteps comprising: a) providing a particulate catalyst support comprisingspheroidal silica particles having an average diameter within the rangeof 5-40 microns and an average effective pore size within the range of50-350 angstroms and comprising at least 50% of the surface area of saidparticulate support within the pore volume of said support; b)contacting said particulate silica support with an oligagenous solutionof an alumoxane co-catalyst in an oligagenous carrier liquid in anamount sufficient to incorporate said alumoxane catalyst within the porespaces of said silica support; c) drying said alumoxane-impregnatedsilica support to remove oligagenous carrier liquid therefrom to leavean alumoxane residue within the pore spaces of said support; and d)thereafter contacting said alumoxane-containing silica support with astereospecific metallocene having two sterically dissimilarcyclopentadienyl ring structures coordinated with a central transitionmetal atom being in a stereorigid relationship relative to saidcoordinating transition metal atom to prevent relative rotation of saidring structures.